Oh, I know, you want to make sure you've thought it through before you do anything. I would say that's understandable - however, I have met way too many people who never get beyond thinking about it. I mean they don't even put their thoughts down on paper in any fashion, orderly or otherwise.
When I talk to people about the Creative Process, I explain that it’s a mental process first and a physical process second. So, yes, you do have to think about what you want first. However, putting it on paper starts to take it from the metaphysical to the physical. It helps you during the time of your mental work to be able to imagine your ideas as a reality. It begins to take on shape and color in your mind.
Putting your ideas on paper takes you out of the comfort zone of your private thoughts. It helps you develop clarity about what you want. This is one of the primary reasons for preparing a business plan. It forces you to put your ideas down in black and white so that any flaws can be spotted. Otherwise, it's easy to "do a little sidestep" [Charles Durning] and set yourself up for failure.